A person steps outside a circle, and is subsequently teleported back inside it (activated in a trigger's On Deactivation field, basically). This could be done simply by moving the unit, say, 5 meters in the direction of the centre of the circle (getPos TriggerName). What math wizardy would I need to perform to be able to do this rather simple operation? I suck at math, so all I basically need is the function, explanations will be useless (I bet it involves tan and sin and all those scary buggers).

It's too late in the day for me to do math (it doesn't come as naturally to me as I'd like ), but you could make use of CBA and BIS functions thus (if in doubt, leave the real math to people who can do it ):

I forgot to say thank you to Mr. Peanut: thank you! That solved it, no problems whatsoever. Now all I'd need would be a command to setdir the player in the exact direction of the centre of the trigger, but that's really a secondary matter after all